Ultramafic and mafic rocks of the Northern Kraka massif (Southern Urals): petrography, mineralogy and geochemistry

The paper presents the results of studies of ultramafic and mafic rocks of the Northern Kraka massif (Southern Urals). Petrographic, mineralogical and geochemical features and conditions of rock formation are characterized. The massif has outcrops of peridotites with low degrees of secondary alteration, which makes its study important for understanding the processes of mineral formation. Petrographic studies were carried out by optical and scanning electron microscopy methods, chemical composition of minerals was determined using an energy-dispersive spectrometer. The bulk composition of rocks was determined by silicate analysis, the content of impurity elements, including rare-earth elements, was determined by ICP-MS (inductively coupled plasma mass spectrometry). Mineralogy and composition of rock-forming and accessory minerals of peridotites and mafic rocks, as well as their bulk macro- and trace-element compositions were described, and discriminative diagrams for mineral species were plotted. Olivine of ultramafic rocks is represented by highly magnesian variety, orthopyroxenes mainly by enstatite, clinopyroxenes of both ultramafic and mafic rocks - by diopside and augite, in the latter it is highly titanic. Feldspars in mafic rocks correspond to labradorite, bytownite, and potassium feldspar. Cr-spinels form a wide variation of Cr-Al compositions, increasing #Cr from lherzolite to chromitite. Mafic rocks from the serpentinite melange of the Northern Kraka periphery are composed of highly titaniferous augite, plagioclase and potassium feldspar, which distinguishes them from typical ophiolitic gabbros. REE contents in both mafic rocks and peridotites show subhorizontal distribution relative to the estimated pyrolite composition, with REE contents in mafic rocks 1-2 orders of magnitude higher than in peridotites. Significant gross contents of alkaline-earth metals in mafic rocks are consistent with a large proportion of leucocratic minerals. Peridotites of the massif are formed as restite from partial melting of the primitive mantle source, which reached values at most 25 %. The mafic rocks are differentiates of melts extracted from peridotite at much lower degrees of melting (about 15 %), which is also evidenced by the high content of sodium and potassium. In accordance with this infer is also the conclusion that the formation of parental melts for mafic rocks occurred at earlier stages of diapir development. This is evidenced by the greater depth of melt extraction, corresponding to the spinel facies, while the final position of peridotites corresponds to the transition from spinel to plagioclase facies. Contribution of the authors: the authors contributed equally to this article. The authors declare no conflicts of interests.

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